This invention relates to a power converter and, in particular, to a multi-output type DC/DC converter for producing a plurality of output voltages using a switch circuit and a control method thereof.
In the manner which is well known in the art, the DC/DC converter is a power converter for converting an input DC voltage into an output DC voltage which is different from the input DC voltage. In addition, there is a case where a plurality of different output DC voltages (which will later be merely also called “output voltages”) are supplied to a plurality of loads for the input DC voltage (which will later be merely also called “input voltage”). Such DC/DC converters for generating a plurality of output voltages are classified into division type DC/DC converters and multi-output type DC/DC converters.
The division type DC/DC converter is a circuit which comprises a plurality of switch circuits which are equal in number to the loads (outputs) and supplies powers (outputs voltages) with the respective loads. On the other hand, the multi-output type DC/DC converter is a circuit for generating a plurality of output voltages using a single switch circuit.
In the multi-output type DC/DC converter, a plurality of loads are supplied with necessary energy by carrying out time division control to divide a switching frequency (or to pre-assign time slots). Such a multi-output type DC/DC converter is disclosed, for example, in Japanese Unexamined Patent Publication Tokkai No. 2004-96816 or JP 2004-96816 A.
In a conventional multi-output type DC/DC converter, by regulating an ON time interval (a duty factor) in the switch circuit for respective loads each switching period (the time slot), each load is supplied with necessary power. In other words, in the multi-output type DC/DC converter, the time slots are pre-assigned with each load and each load is supplied with necessary power within each time slot. Accordingly, it is necessary to make a current flowing in an inductor zero until an end time point of each time slot. Otherwise, magnetic energy left in the inductor is released in another load at the next time slot.
As described above, an operation mode making the multi-output DC/DC converter so as to make the current flowing in the inductor zero before switching from a time slot to the next time slot is called a “current discontinuous mode” in the art. On the other hand, another operation mode making the multi-output DC/DC converter in a state where the current flowing in the inductor does not become zero at a time instant when a time slot is switched to the next time slot is called a “current continuous mode” in the art.
In the conventional multi-output type DC/DC converter, there is a phenomenon such that the current flowing in the inductor does not become zero in an abnormal condition where any of the loads becomes an over-load. In order to resolve this problem, the above-mentioned JP 2004-96816 A discloses a technical idea which can make the current flowing in the inductor zero at the abnormal condition such as an over-load state. However, it is disadvantageous in that a controller has a complicated structure because a complicated control is required to realize the technical idea.
In addition, a switching power supply circuit or a switching regulator using a single inductor is known, for example, in U.S. Pat. No. 6,900,620 issued by Nishimori et al. In the switching power supply circuit, time slots are pre-assigned with loads. It will be assumed that the loads are equal in number two. In this even, the time slots are alternately assigned the two loads. The switching power supply circuit regulates, in accordance with weight of the loads, a time interval (duty factor) necessary for turning a main switch on in order to flow a current from an input power supply to the single inductor. In addition, according to Nishimori et al., an embodiment in which energy filling time intervals for the two loads are different from each other is disclosed.
Furthermore, a two-output type DC/DC conversion circuit, which controls output voltages so as to keep constant by controlling an operation duty of a switching element using a single DC reactor, is disclosed, for example, in Japanese Unexamined Patent Publication Tokkai Hei No. 11-168876 or JP 11-168876 A. Accordingly, in a two-output type converter, an ON time interval (duty factor) of the switching element varies in accordance with weight of the loads.
In addition, a multiple output back converter using a single inductor to control turning on/off of switching means by a pulse width modulator is disclosed, for example, in U.S. Pat. No. 6,222,352 issued by Lenk. In the back converter, voltage outputs are regulated and controlled by controlling a duty cycle and an ON time interval of the switching means.
Disclosed in the above-mentioned patent documents, any multi-output type DC/DC converter regulates an ON time interval for turning switching means (a switch for flowing a current from an input power supply to an inductor) on in accordance with weight of loads. Accordingly, any multi-output type DC/DC converter, which is disclosed in the above-mentioned patent documents, can operate only at the current discontinuous mode.